Raster output scanning printers have been developed to respond to a multiplicity of digital signals providing print instructions. A raster output scanning printer traces a raster scan similar to a television raster. The mechanism for tracing the raster scan and for providing impulses which are sensed by some image forming medium are conventionally termed the printer engine. One example of a conventional raster output scanning printer engine is a laser scanning printer in a xerographic copy reproduction system. In such a xerographic printer, a laser beam is transmitted and optically modulated to provide a beam of light directed at a rotating polygon driven by a motor and having mirrored surfaces. Instant retracing occurs each time a different mirror surface arrives in position. The laser beam is reflected successively from each mirrored surface of the polygon at an angle that changes with rotation of the polygon. The beam thus is reflected in a scanning motion and is directed at a rotating xerographic copy drum. The beam is gated to strike the drum or blocked from striking the drum dependent upon digital print information which controls the vector of beam transmission. The result is that a latent image is produced on the drum wherein charged areas are either discharged or not, depending upon the binary information that either directs the beam at the drum or deflects the beam to prevent it from reaching the drum. Thereafter, toner is contacted with the area of the drum containing the latent image. Charges on the drum in the latent image area are transferred to sheets of paper, upon which the toner then becomes imprinted.
The successive binary bits of information controlling the vector of the laser beam are generated in some type of electronic image processor, usually a character generator. Heretofore, electronic image processors have been directly connected to raster output scanning printers in an integral inseparable fashion. As a result, the equipment configurations employed with raster output scanning printer systems are quite inflexible. Defects in system components or connections must be remedied at the installation site. Use of either the electronic image processor or the raster output scanning printer with any alternative device is virtually prohibited because of the intricate interconnection of these devices.
It is an object of the present invention to provide an adapter for a particular raster output scanning printer that responds to video and control information synchronously provided in binary form from any one of several types of electronic image processor. This is achieved by construction of the adapter with specific controls that respond to command and video information provided on an ordered array of dedicated input lines. In the preferred embodiment the adapter interfaces to a nine line plug connection with interface jacks positioned according to a standard raster output scanner interface convention. Each of the lines is dedicated to the provision of particular types of information. Preferably nine signal lines of connections are provided in a single plug in a specified order and at predetermined special intervals to interface with the adapter. Each signal line is a differentially driven, twisted pair of wires which may or may not be shielded. The adapter is thereby suitable for use to allow the raster output scanner associated therewith to be driven by any electronic image processor constructed with outputs according to this convention. This design allows the adapter to be used with raster output scanner units that require a bit rate of between one and one hundred megabits per second. The adapter is designed to allow some degree of physical separation of the adapter and raster output scanner associated therewith from the electronic image processor. A maximum allowable physical separation is about 40 meters for a 100 megabit per second system and 400 meters for a 10 megabit per second system.
A related object of the invention is to provide flexibility in the use of particular raster output scanning printers. A printer employing the adapter of the invention may be operated according to inputs from electronic image processors of varying degrees of sophistication. The printer may be subjected to particular specialized commands, if the nature of the utilization of the printer warrants. Alternatively, a more economical electronic image processor may be employed if specialized commands are not needed.
A further object of the invention is to provide an interface for a raster output scanner that performs veracity checks on data provided by the electronic image processor. Because of the typical situation in which there is a physical separation between the electronic image processor and the raster output scanner, noise externally induced in the signal transmission cable frequently becomes a problem. To prevent the printer from responding to such noise, veracity checks are performed on the binary bits transmitted by the electronic image processor. The state of these bits is sampled at intermediate times during a prescribed bit duration. Sampling may occur twice in order to detect noise of bits of command information, and may be performed three times within the bit interval to verify the start of a command from the electronic image processor.
Another object of the invention is to provide a means for informing an electronic image processor of the status of various operating conditions, both within the adapter of the invention and also in the raster output scanning printer. Status conditions within the printer and adapter, and also from externally connected equipment if desired, are signalled as being either normal or abnormal. These signals are stored in status buffers which are multiplexed through the adapter to a single status line connected to the electronic image processor. The adapter polls the various status buffer locations rapidly in predetermined order to provide status information to the electronic image processor. Without this feedback from the adapter, the adapter is insensitive to malfunctions of the printer or related equipment and operates to transmit information to the printer whether or not the printer receives and responds to that information.
A very significant feature of the adapter of the invention is the ability to respond to fluctuations of motor speed occurring in a motor driven printer engine. In a raster output scanning laser printer, the speed of the motor driving the mirror surfaced polygon, although closely controlled, does fluctuate slightly. In conventional systems this produces printed copies in which the spacing between characters and portions of characters is either to great or to small. That is, the printed material will be compressed too closely together if the motor speed slows slightly, or the characters and spacing therebetween will be stretched if the motor rotates slightly above the desired speed. The copy produced in conventional systems contains printed material with a very significant number of such irregularities. Most conventional systems attempt to adjust motor speed to the rate of provision of binary data. However, because of the much greater speed of data provision as compared with the speed of operation of the analog components that adjust motor speed, such regulation is inadequate.
It is an object of the present invention to adjust the speed of data provision to the raster output scanning printer to accomodate momentary fluctuations in printer engine speed. Since the digital components that regulate the provision of data arriving from the electronic image processor operate many orders of magnitude faster than the analog components governing motor speed, the adapter easily allows the rate of data provision to the printer to be adjusted to accomodate fluctuations in print motor speed.